Chapter VI Properties of Dry Gases

CHAPTER VI:

PROPERTIES OF DRY GASES

DEFINITION

• A gas is defined as a homogeneous fluid of low viscosity and density that has no definite volume but expands to completely fill the vessel in which it is placed.

• The natural gas is a mixture of hydrocarbon (methanes, ethanes, propanes, butanes, pentanes, and small amounts of hexanes and heavier)and nonhydrocarbon gases (carbon dioxide, hydrogen sulfide, and nitrogen).

• Gas reservoirs: If the reservoir temperature is greater than the critical temperature of the hydrocarbon fluid, the reservoir is considered a gas reservoir

• Type of gas reservoir:1. Retrograde gas2. Wet Gas3. Dry Gas

PROPERTIES OF DRY GASES

1. Gas Formation Volume Factor (Bg)2. Isothermal Gas Compressibility

Coefficient (Cg)3. Viscosity of Gas (µg)4. Heating Value (Lc)

Gas Formation Volume Factor (Bg)

• Gas formation volume factor (Bg) is the volume of gas at reservoir conditions required to produce one standard cubic foot of gas at the surface (Res. cuft/scf).

• Bg is the ratio of the volume occupied at reservoir temperature and pressure by a certain weight of gas to the volume occupied by the same weight of gas at standard conditions.

Gas Formation Volume Factor (Bg)

conditiondardsgas

reservoirgasg V

VB

tan@

@

Conversion

Isothermal Gas CompressIbIlIty CoeffiCient (Cg)

• Defined as the change in volume per unit volume for a unit change in pressure.

For real gas

• The Law of Corresponding states can be used to express the above equation in another form:

Combining this

equati

on with

equati

on above yiel

ds:

(cpPpc) is called pseudo reduced compressibility (cpr)

Viscosity of Gas (µg)

• Viscosity is a measure of the resistance to flow.• The viscosity term is called dynamic viscosity

whereas kinematic viscosity is the dynamic viscosity divided by the density.

• It is given in units of centipoise. A centipoise is a gm/100 sec.cm

• Kinematic viscosity has units of cm2/100 sec and the term is called centistoke.

Viscosity of ethane

Viscosity of paraffin hydrocarbon gases at one atmosphere

Viscosity of gases at atmospheric pressure

Viscosity ratio vs pseudo reduced temperature and pseudo pressure

Viscosity of mixtures

HEATING VALUE

• Heating value of a gas is the quantity of heat produced when the gas is burned completely to carbon dioxide and water.

• Unit: British thermal unit per standard cubic foot of gas (BTU/scf).

• Terms in Heating value:1. Wet (gas saturated with vapor about 1.75%V).2. Dry or bone dry (gas contain no vapor before heating).3. Gross heating value or total heating value (heat produced

in complete combustion under constant pressure with the combustion products cooled to standard conditions & the water in the combustion products condensate to the liquid state).

4. Net heating value (similar but no water of the combustion remains vapor in standard conditions).

• In petroleum the gross heating value (dry) is frequently used. Means complete combustion of a bone-dry gas with the water produced during combustion condensed to liquid.

Heating value (Lc) calculation

• For ideal gases:

• For natural gases (real gases):

• Compressibility factor (z) calculated from Appendix A.

conversion

• Net heating value (dry) converted into net heating value (wet).

• Gross heating value (dry) converted into gross heating value (wet).